Plastic objects now play a major part in modern life. Such objects are marked on their surface for information and decoration. Often the plastic is handled or subject to forces of abrasion or other types of wear, which could adversely affect any surface markings. Such damage could reduce the effectiveness of any such markings.
Ink jet is a modern method of marking, which has had a large degree of success in the market for a variety of reasons. It is a non-impact printing process, whereby ink is squirted through very fine nozzles and the resultant ink droplets form an image directly on a substrate. One advantage of the process is that the equipment can be made and sold relatively inexpensively and for this reason it has been adapted for sale in the home as the prime method of printing from PC's (Personal Computers). Another advantage is that because it is a non-impact process, printing can be done on fragile surfaces such as those of eggs. As the process does not work through a master, it can be used to print variable information, where each print is different from the previous one. Working through a master means that once the master is imaged, the image is fixed for as many impressions as required. After making the required number of impressions, the master is discarded and a new one made for the next job.
There are two main types of ink jet process. In one process, usually termed continuous ink-jet printing (CIJ), a stream of ink drops are electrically charged and then are deflected by an electric field, either directly or indirectly, onto the substrate. In the second process, usually called Drop on Demand (DOD) ink-jet printing, the ink supply is regulated by an actuator such as a piezoelectric actuator. The pressure produced during the actuation forces a droplet through a nozzle onto the substrate. CIJ inks need to have some electrical conductivity, but inks for DOD ink-jet printing do not need to be conductive.
In all of the ink jet processes, in order to form suitable droplets, the ink must have a relatively low viscosity during the actual jetting process. This is generally in the region of 1 to 30 centipoise. This can be achieved by using a low viscosity carrier fluid, which may be water, or volatile organic liquid, or a relatively non-volatile organic liquid. Alternatively, the ink can be heated up to lower its viscosity during jetting. For many reasons, water has been found to be a suitable carrier liquid. It is inexpensive, readily available, environmentally harmless and has a high surface tension, which enables additives needed for the inkjet process to be used, as it is easier to find useful additives to reduce surface tension than to increase it.
Ink jet writing on plastic presents specific problems that inventors have tried to solve. One such approach is to use inks other than aqueous based, which can be made to dry and bond to plastic. WO 97/27053 by Jennel et al describes the use of inkjet to digitally write on packaging material. The printing can be done directly onto a pre-formed bottle such as one made from PET (polyester), or onto a carton blank or a web of packaging material. The invention is claimed to provide an advanced level of automation with minimum operator intervention. In order to achieve good adhesion to materials such as PET, ultra-violet (UV) sensitive inks are used and after jetting they are cured by UV radiation. The ink jet head is DOD and described as one supplied by the company Spectra. This is the most widely accepted way of using UV curing inkjet inks, as the alternative method, CIJ, generally uses water based inks and the inks must contain electrically conductive material. UV sensitive inks are generally based on organic acrylate mixtures that do not contain electrically conductive ingredients and are therefore less easily adapted for use in CIJ.
UV sensitive inkjet inks are more expensive than water-based inks and will remain so because, by definition, water-based inks contain a large quantity of water, which is relatively inexpensive. Because with UV inkjet inks all of the jetted material remains on the substrate surface (where the substrate is impermeable) inks are deposited in the form of tiny hemispherical structures. Process color work, where three or four separate inks are applied over the same area, can thus have a Braille-like feeling. Such an effect limits print quality.
The use of water-based inks on plastics would be advantageous for several reasons. As has been mentioned above, they have cost advantages; they can be used in both DOD and CIJ inkjet systems and they do not pile-up because the major part of the inkjet drop is water, which disappears either by absorption, if the substrate is pervious, or by evaporation, or both. However, there are a number of problems with using water-based inkjet inks on plastics. They have wetting problems with relatively low energy plastic surfaces (for instance that of PET) as well as slow drying, which for non-absorbent plastic surfaces has to occur only by evaporation. Also, they have low wet smear resistance—i.e. after they have dried, they can be easily smeared with a wet finger. These problems have been appreciated for some years and for instance U.S. Pat. No. 3,889,270 seeks to address this problem by using for instance polyvinyl alcohol as a coating for the substrate and including for instance hydrophilic silica gel. U.S. Pat. No. 4,269,891 uses for instance polyvinyl alcohol and pigments such as titanium dioxide for suitable substrate coatings. U.S. Pat. No. 4,474,850 uses for instance salts of high molecular weight carboxylic acids and locks the aqueous dye based inks into the surface by ionic interaction. U.S. Pat. No. 4,474,850 is concerned with the production of transparencies. U.S. Pat. No. 4,592,951 expresses the need to cross-link the polyvinyl alcohol because uncross-linked layers are generally too tacky. Jones, in U.S. Pat. No. 4,649,064 employs hydrophilic film, partially cross-linked, onto which a cross-linkable ink jet ink is jetted. The ink drying process is at room temperature. In general, in order to avoid the need for customers to have special drying units, drying is expected to occur without any additional prompting with energy such as heat and thus it is at room temperature. The ink is then cross-linked with an agent within the substrate coating. WO 99/21724 by Wang et als. addresses the problem of ink smearing. The patent application describes the use of two layers—an inner non-cross-linked hydrophilic coating and an outer cross-linked hydrophilic coating. In one embodiment, an inkjet image is applied before curing to avoid wet smear. Similarly, US 2001/0036552 by Otani et al. describes coating a substrate with two layers for water-based pigment inks to give better colors and image fastness.
U.S. Pat. No. 5,537,137 addresses the problems of aqueous inkjet printing on plastics by introducing a reactive material into the substrate coating and heating after depositing a reactive inkjet ink onto the coated substrate. The reaction fixes the ink into the coating by the reaction of the ink with the coating and also cross-links the coating itself to increase the coating durability. The disadvantages of this approach are that the presence of a reactive system, which cures the background areas, limits shelf life of any such coated product before imaging, as the cross-linking reactions that occurs rapidly at high temperatures also proceed slowly at room temperature. Also, inks used are limited to those containing reactive species either as a dye or pigment dispersion or as a polymeric material which serves as the pigment dispersant. If the energy used for fixing the ink is ultraviolet light, then the media must be protected from such light during handling. This may necessitate yellow light where the printing process occurs. The resultant prints of such a process will show variable gloss on the surface, reflecting the affect of the variations in absorption of water, dependent on the amount of ink deposited in the variable areas of the printed material. An alternative approach for achieving smear-proof images is to overcoat the ink jet image. U.S. Pat. No. 6,095,050 describes the use of organic lacquers to fix aqueous ink jet inks into uncoated substrates where the ink jet ink is still wet during the overcoating process. This method works with paper where ink absorption into the substrate enables smudging of wet on wet to be avoided.
Much of the work for aqueous ink jet printing on plastic has been addressed to achieving smear-proof properties for the purposes of handling, for applications such as the imaging of overhead transparencies. The present invention addresses more stringent requirements for handling, where the printed object may be subject to handling and solvents, which would damage ink jet images which were merely smear proof. The method and materials below described for working the present invention provide the means of producing information or decoration onto plastics using aqueous ink jet inks, resulting products being of excellent image quality as well as being smear proof and abrasion resistant and generally having a high resistance to physical and chemical damage, appropriate to the present applications. Applications for such methods and materials include cards used for ID's or for smart card applications, where the product must undergo handling and the unprinted material should benefit from a long shelf life. A further application is in the production of printing on bottles, whereby white substrate coating onto plastics such as PET (polyester) can be used to provide areas on which aqueous ink jet inks can be jetted followed by a protective overcoat, as well as providing the contrasting background for the transparent ink jet inks.